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This reactor generates spent fuel rods laced with plutonium and is the core of the Yongbyon nuclear complex.

The reactor, when fully running, is capable of producing enough plutonium to produce one atomic bomb a year.

The move will boost fears in the US and among its allies about North Korea's push for nuclear-tipped missiles that can reach the US, technology it is not currently believed to have.

North Korea conducted its third nuclear test in February, prompting UN sanctions that have infuriated its leaders and led to the current tensions.

The country has since declared that making nuclear arms and a stronger economy are the nation's top priorities.

North Korea added the 5-megawatt, graphite-moderated reactor to its nuclear complex in 1986 after seven years of construction.

The country began building a 50-megawatt and a 200 megawatt reactor in 1984, but their construction was suspended under a 1994 nuclear deal with the US.

North Korea has long said that the reactor operation is aimed at generating electricity.

It takes about 8,000 fuel rods to run the reactor. Reprocessing the spent fuel rods after a year of reactor operation could yield about 7kg of plutonium, which experts say is enough to make at least one nuclear bomb.